In recent years, HDTV (High Definition Television) resolution has been used in broadcasting, package media and other video sources. At the same time, audio and visual technologies are booming.
For the above HDTV sources, sound is recorded in a more sophisticated way than a conventional one. For example, at a sampling frequency of 96 kHz or 192 kHz and with a dynamic range of 24 bits, sound is more accurately recorded.
However, in the kinds of acoustic room in a normal house, such as a living room, audio room or audio visual room, that have limits in size or design, it has been difficult to obtain an excellent sound field.
The following are among the factors that hamper the creation of an excellent sound field in a living room and other kinds of acoustic room: multiple-diffraction reflection (flutter echo), standing wave, and vibration of a wall caused by excess low-pitched sound. Multiple-diffraction reflection emerges between wall surfaces of a room, which face each other. Standing waves occur in a room for the same reason. The vibration of walls is caused by excessive low-pitched sound.
In designing a living room and other kinds of acoustic room, reducing the above factors that hamper the creation of an excellent sound field is important.
To obtain an excellent sound field, adjusting an early-reflected sound or reverberant sound is also important. That is, what is important is to enable the spreading of sound to be felt even in a room with a limited capacity by adjusting an early-reflected sound or reverberant sound.
To adjust sound in a room, acoustic panels made of a sound-absorption material have been available in the market. Such acoustic panels are placed in a room or attached to a wall or ceiling for use.
What is disclosed in Patent Document 1 as a conventional acoustic panel is a sound diffusion panel that makes it easier to adjust a horizontal sound environment (which is referred to as Conventional Technology 1, hereinafter).
The acoustic panel of Conventional Technology 1 includes two supporting legs, which stand at predetermined intervals, and two acoustic panels, which are supported by the supporting legs. In the acoustic panel, the two acoustic panels are connected together so as to freely open in the horizontal direction. The acoustic panels are each supported by the supporting legs via a slide mechanism that slides in the horizontal direction. On each supporting leg, a rotating mechanism is provided to rotate the acoustic panels in the horizontal direction. On the front sides of the acoustic panels, working faces are formed to reflect and absorb sound, with the back sides of the acoustic panels supported by the supporting legs.
When the acoustic panel of Conventional Technology 1 is used, it is possible to make fine adjustments to the direction of sound absorption and reflection by opening and closing the acoustic panels as if to open and close a folding screen. Since the acoustic panels are supported by the supporting legs via the slide mechanism and the rotating mechanism, the acoustic panels open and close smoothly. Therefore, flutter echoes and standing waves can be reduced by making appropriate adjustments to angles and installation sites.
In the meantime, with reference to a side view shown in FIG. 25 and a top view in FIG. 26, a relationship between direct sound and early-reflected sound in sound fields of various kinds of acoustic room will be described. The early-reflected sound refers to a primary reflected sound from a floor, wall, ceiling or the like. The diagrams each show the relationship between direct sound and early-reflected sound.
A sound wave that occurs at a speaker P (sound generation section) shown in FIG. 25 or 26 reaches a listener L as a direct sound 1200. In addition, an early-reflected sound reaches the listener L after being emitted from the speaker P. The diagrams each show an example of early-reflected sounds: a ceiling reflection sound 1320, a floor reflection sound 1330, a sidewall reflection sound 1350, a front-wall reflection sound 1310, and a rear-wall reflection sound 1340. The ceiling reflection sound 1320 is a primary reflected sound that is reflected off the ceiling. Similarly, the floor reflection sound 1330 is a primary reflected sound that is reflected off the floor. Similarly, the sidewall reflection sound 1350 is a primary reflected sound that is reflected off the left-side and right-side walls. Similarly, the front-wall reflection sound 1310 is a primary reflected sound that is emitted from a bass reflex port or the like and reflected off the wall ahead of the listener L. Similarly, the rear-wall reflection sound 1340 is a primary reflected sound that is reflected off the wall behind the listener L. Besides the above sounds, there are sound waves, called reverberant sound, which occur after the above reflected sounds are further reflected off the walls of the room and attenuated before reaching the listener L.
With reference to FIG. 27, the relationships between the above sound waves will be described.
When compared with the direct sound 1200 that has the highest sound pressure, an early-reflected sound 1300 arrives after going a longer distance due to reflection. Therefore, in terms of time, the early-reflected sound 1300 arrives later than the direct sound. Moreover, a repeatedly-reflected reverberant sound 1400 also arrives.
The listener L listens to the sound waves and can recognize acoustic environments, or sound fields, of various kinds of acoustic rooms consciously or unconsciously.
Accordingly, the listener L may feel the confine of the various kinds of acoustic room by the intensity and direction of the early-reflected sound or the volume of reverberant sound.
Therefore, what is important is to make sure that the listener can feel the spreading of sound even in a room with a limited capacity by adjusting the early-reflected sound or reverberant sound in order to obtain an excellent sound field.
For the above purpose, acoustic panels made of a sound-absorption material have been available in the market: The acoustic panels are placed in a room or attached to a wall or ceiling to adjust sound in the room.
What is disclosed in Patent Document 2 as a conventional acoustic panel is a sound diffusion panel that makes it easier to adjust a horizontal sound environment (which is referred to as Conventional Technology 2, hereinafter).
The acoustic panel of Conventional Technology 2 includes two supporting legs, which stand at predetermined intervals, and two acoustic panels, which are supported by the supporting legs. In the acoustic panel, the two acoustic panels are connected together so as to freely open in the horizontal direction. The acoustic panels are each supported by the supporting legs via a slide mechanism that slides in the horizontal direction. On each supporting leg, a rotating mechanism is provided to rotate the acoustic panels in the horizontal direction. On the front sides of the acoustic panels, working faces are formed to reflect and absorb sound, with the back sides of the acoustic panels supported by the supporting legs.
When the acoustic panel of Conventional Technology 2 is used, it is possible to make fine adjustments to the direction of sound absorption and reflection by opening and closing the acoustic panels as if to open and close a folding screen. Since the acoustic panels are supported by the supporting legs via the slide mechanism and the rotating mechanism, the acoustic panels open and close smoothly.
Therefore, making adjustments to the early-reflected sound or reverberant sound results in an improvement in the sound fields of various kinds of acoustic rooms. Moreover, it is possible to reduce multiple-diffraction reflection (flutter echo), which occurs between the wall surfaces of a room that face each other, standing waves, vibration of the walls, and the like.
Meanwhile, from the back side of a speaker, such as a dynamic speaker that outputs sound with the help of vibration of a diaphragm, a sound wave is emitted in such a way that the phase difference between the sound wave and a sound wave emitted from the front side of the speaker is one-half of the wavelength. If the back-side sound wave goes around the speaker to the front side and interferes with the speaker's front-side sound wave, the intensity of sound from the speaker is attenuated.
In general, to prevent the front-side sound from being attenuated by the speaker's back-side sound wave, the speaker is attached to one surface of a speaker cabinet that is in the shape of a rectangular parallelepiped, so that the speaker's back-side sound is contained in the cabinet, preventing the back-side sound wave from going around to the front side of the speaker.
However, if the speaker's back-side sound is emitted into the cabinet, the sound wave reflects off the inner surfaces of the wall portions of the cabinet. The reflection of the reflected sound wave between the following inner surfaces causes a standing wave whose antinodes of vibration are positioned at the inner surfaces of the wall portions facing each other: the inner surfaces of a front-wall portion (baffle board) to which the speaker is attached and a rear-wall portion that faces the front-wall portion, the inner surfaces of upper- and lower-wall portions, and the inner surfaces of right-side and left-side wall portions.
Therefore, the problem is that the standing wave hampers the vibration of the diaphragm of the speaker. The intensity of the standing wave varies according to the frequency. Because of the standing wave, the speaker has a “flaw” in the frequency characteristics, i.e., only a specific frequency is amplified or attenuated. Thus, the frequency characteristics become worse.
A simple, conventional way to address the above problem is to put a sound-absorption material, such as glass wool, onto the inner surfaces of the wall portions of the cabinet in order to reduce the standing wave. However, at the same time, the sound-absorption material could damage natural tone of the sound and cause other unpleasant effects.
What is disclosed in Patent Document 3 is a technique of removing standing waves by placing a plurality of flat or curved division plates in a speaker cabinet and disposing the division plates on the inner surfaces, which face each other, of the upper, lower, left-hand, right-hand, front and rear wall portions of the cabinet in such a way that the division plates do not run parallel to the inner surfaces (which is referred to as Conventional Technology 3, hereinafter).